Hi Bob,
Have a look at
www.kbkickboxing.co.uk and see what you think. This is where I train and I couldn't ask for it to be any better. There are a whole load of classes, so you can attend either once a week (which is £35 a month, and you can 'make up' any classes you miss and also get three extra classes per two months that you can take when you want included in the price) or go to as many classes as you like for £70 a month. I think the taster classes for beginners are a bit cheaper.
Classes are located in a few places in central London, including Euston and Tottenham Court Road. In theory for £70 a month you could do 12 classes a week! You start off wearing whatever you're comfortable in, and then only when you know that you're going to stick with it do you have to invest in equipment.
I started off going once a week to learn the basics, but soon discovered that it wasn't often enough and switched to going as much as I like (averaging 5 hours a week). I've made some great friends, learnt the meaning of respect, am fitter than I've ever been in my entire life, and absolutely love it.
Hope this helps,
Jelly